1. Field of the Invention
The present invention relates to a photosensor system having a photosensor array constituted by two-dimensionally arraying a plurality of photosensors, and a drive control method in the photosensor system.
2. Description of the Related Art
One of conventional two-dimensional image reading apparatuses for reading printed matter, a photograph, or a fine three-dimensional shape such as a fingerprint is a photosensor system having a photosensor array constituted by arraying photoelectric converting elements (photosensors) arranged in a matrix.
As well known, a CCD has a structure in which photosensors such as photodiodes or thin film transistors (TFT: Thin Film Transistor) are arranged in a matrix, and the amount of electron-hole pairs (the amount of charge) generated corresponding to the amount of light entering a light receiving section of each photosensor is detected by a horizontal scanning circuit and vertical scanning circuit to detect the luminance of radiation.
In a photosensor system using such a CCD, it is necessary to respectively provide selective transistors for causing the scanned photosensor to assume a selected state. This increases the system size as the number of pixels increases.
In place of the combination of the photosensor and the selective transistor, a photosensor (to be referred to as a double-gate photosensor hereinafter) is now being developed, which is formed of a thin film transistor having a so-called double-gate structure and has both a photosensing function and a selecting function.
FIG. 26A is a sectional view showing the structure of a double-gate photosensor 10. FIG. 26B is a circuit diagram showing the equivalent circuit of the double-gate photosensor 10.
The double-gate photosensor 10 comprises a semiconductor thin film 11 formed of amorphous silicon or the like, n+-silicon layers 17 and 18 respectively formed at the two ends of the semiconductor thin film 11, source and drain electrodes 12 and 13 respectively formed on the n+-silicon layers 17 and 18, a top gate electrode 21 formed above the semiconductor thin film 11 via a block insulating film 14 and upper gate insulating film 15, a protective insulating film 20 provided on the top gate electrode 21, and a bottom gate electrode 22 provided below the semiconductor thin film 11 via a lower gate insulating film 16. The double-gate photosensor 10 is provided on a transparent insulating substrate 19 formed of glass or the like.
In other words, the double-gate photosensor 10 includes an upper MOS transistor comprised of the semiconductor thin film 11, source electrode 12, drain electrode 13, and top gate electrode 21, and a lower MOS transistor comprised of the semiconductor thin film 11, source electrode 12, drain electrode 13, and bottom gate electrode 22. As is shown in the equivalent circuit of FIG. 26B, the double-gate photosensor 10 is considered to include two MOS transistors having a common channel region formed of the semiconductor thin film 11, TG (Top Gate terminal), BG (Bottom Gate terminal), S (Source terminal), and D (Drain terminal).
The protective insulating film 20, top gate electrode 21, upper gate insulating film 15, block insulating film 14, and lower gate insulating film 16 are all formed of a material having a high transmittance of visible light for activating the semiconductor layer 11. Light entering the sensor from the top gate electrode 21 side passes through the top gate electrode 21, upper gate insulating film 15, and block insulating film 14, and then enters the semiconductor thin film 11, thereby generating and accumulating charges (positive holes) in a channel region therein.
FIG. 27 is a schematic view showing a photosensor system constituted by two-dimensionally arraying double-gate photosensors 10. As shown in FIG. 27, the photosensor system comprises a sensor array 100 that is constituted of a large number of double-gate photosensors 10 arranged in an n×m matrix, top and bottom gate lines 101 and 102 that respectively connect the top gate terminals TG and bottom gate terminals BG of the double-gate photosensors 10 in a row direction, top and bottom gate drivers 110 and 120 respectively connected to the top and bottom gate lines 101 and 102, data lines 103 that respectively connect the drain terminals D of the double-gate photosensors 10 in a column direction, and an output circuit section 130 connected to the data lines 103.
In FIG. 27, φtg and φbg represent control signals for generating a reset pulse φTi and readout pulse φBi, respectively, which will be described later, and φpg represents a pre-charge pulse for controlling the timing at which a pre-charge voltage Vpg is applied.
In the above-described structure, as described later, the photosensing function is realized by applying a predetermined voltage from the top gate driver 110 to the top gate terminals TG, while the readout function is realized by applying a predetermined voltage from the bottom gate driver 120 to the bottom gate terminals BG, then sending the output voltage of the photosensors 10 to the output circuit section 130 via the data lines 103, and outputting serial data Vout.
FIGS. 28A to 28D are timing charts showing a drive control method of the photosensor system, and showing a detecting period (i-th row processing cycle) in the i-th row of the sensor array 100. First, a high-level pulse voltage (reset pulse; e.g., Vtgh=+15V) φTi shown in FIG. 28A is applied to the top gate line 101 of the i-th row, and during a reset period Trest, reset operation for discharging the double-gate photosensors 10 of the i-th row is executed.
Subsequently, a bias voltage φTi of low level (e.g., Vtgl=−15V) is applied to the top gate line 101, thereby finishing the reset period Trest and starting a charge accumulating period Ta in which the channel region is charged. During the charge accumulating period Ta, charges (positive holes) corresponding to the amount of light entering each sensor from the top gate electrode side are accumulated in the channel region.
Then, a pre-charge pulse φpg shown in FIG. 28C with a pre-charge voltage Vpg is applied to the data lines 103 during the charge accumulating period Ta, and after a pre-charge period Tprch for making the drain electrodes 13 keep a charge, a bias voltage (readout pulse φBi) of high level (e.g., Vbgh=+10V) shown in FIG. 28B is applied to the bottom gate line 102. At this time, the double-gate photosensors 10 are turned on to start a readout period Tread.
During the readout period Tread, the charges accumulated in the channel region serve to moderate a low-level voltage (e.g., Vtgl=−15V) which has an opposite polarity of charges accumulated in the channel region and is applied to each top gate terminal TG. Therefore, an n-type channel is formed by the voltage Vbgh at each bottom gate terminal BG, the voltage VD at the data lines 103 gradually reduces in accordance with the drain current with lapse of time after the pre-charge voltage Vpg is applied. More specifically, the tendency of change in the voltage VD at the data lines 103 depends upon the charges accumulating period Ta and the amount of received light. As shown in FIG. 28D, the voltage VD tends to gradually reduce when the incident light is dark, i.e., a small amount of light is received, and hence only small charges are accumulated, whereas the voltage VD tends to suddenly reduce when the incident light is bright, i.e., a large amount of light is received, and hence large charges are accumulated. From this, it is understood that the amount of radiation can be calculated by detecting the voltage VD at the data lines 103 a predetermined period after the start of the readout period Tread, or by detecting a period required until the voltage VD reaches a predetermined threshold voltage.
Image reading is performed by sequentially executing the above-described drive control for each row of the sensor array 100, or by executing the drive control for each row in a parallel manner at different timings at which the driving pulses do not overlap.
Although the photosensor system adopts the double-gate photosensor as a photosensor in the above description, even a photosensor system using a photodiode or phototransistor as a photosensor has operation steps: reset operation→charge accumulating operation→pre-charge operation→reading operation, and uses a similar drive sequence. The conventional photosensor system as above has the following problems.
In this photosensor system, such a photosensor array is formed on one surface of the transparent substrate such as a glass substrate, as described above, and a light source is provided on the back surface side of the transparent substrate. Light emitted by the light source irradiates a subject (finger or the like) placed above the photosensor array. The reflected light corresponding to the image pattern of a fingerprint or the like is received and detected as brightness information by each photosensor, reading the subject image. Image reading operation of the photosensor array detects brightness information on the basis of the amount of charges accumulated in each photosensor during a period corresponding to a set image reading sensitivity (charge accumulating period for the double-gate photosensor).
In the photosensor system using the above-described photosensor, factors including an environmental illuminance in a use place such as an indoor or outdoor place and the type of subject change depending on a use environment. To read a subject image in various use environments, the image reading sensitivity of the photosensor must be properly adjusted.
The proper image reading sensitivity of the photosensor changes depending on ambient conditions such as an environmental illuminance. In the prior art, therefore, a circuit for detecting the environmental illuminance must be additionally arranged. Alternatively, reading operation is done for a standard sample placed on the sensing surface before the start of normal image reading operation, while the reading sensitivity is changed to a plurality of values. An optimal image reading sensitivity corresponding to ambient conditions such as the environmental illuminance is obtained and set on the basis of detection result or reading result. However, the above-described prior art suffers the following problems.                (1) When the photosensor system is applied to a fingerprint reading apparatus or the like, the state of the skin surface layer of a finger (or human body) serving as a subject varies depending on the gender and age of the person, the individual difference such as the physical condition, or an external environment such as a temperature or humidity. This inhibits setting a proper image reading sensitivity when the image reading sensitivity is set based on reading operation before the start of normal image reading operation. For this reason, the apparatus malfunctions in fingerprint collation processing or the like.        
More specifically, if the skin surface layer of the finger as a subject is keratinized, the brightness of the ridge pattern of the keratinized fingerprint is observed higher than that of a non-keratinized normal skin surface. The brightness difference detected by the photosensor becomes larger than an original value. If the image reading sensitivity is set based on this brightness information, the image reading sensitivity is set to a lower value than an originally appropriate value. As a result, a subject image such as a fingerprint cannot be accurately read, decreasing the collation precision of the fingerprint.                (2) If a foreign substance deposited on the sensing surface of the photosensor or a defect is generated in the photosensor element in reading operation before normal image reading operation, the direct use of a read result containing an abnormal value causes a failure in setting a proper image reading sensitivity. This inhibits accurate reading operation of a subject image. When this photosensor system is applied to a fingerprint reading apparatus, the apparatus may malfunction in fingerprint collation processing.        